The present invention relates to a fuel supply apparatus for an internal combustion engine that pressurizes fuel with a high-pressure pump and discharges the fuel from the pump into a high-pressure pipe for supplying high-pressure fuel to an in-cylinder injector.
Japanese Laid-Open Patent Publication No. 7-103048 discloses a conventional fuel supply apparatus for an internal combustion engine. The conventional fuel supply apparatus is applied to an internal combustion engine that includes an in-cylinder injector and an air-intake passage injector in each of its cylinders. The internal combustion engine normally activates an appropriate one of the above two types of injectors to inject fuel according to the engine driving state, such as the engine load and the engine speed. When fuel is to be injected from the in-cylinder injector (in-cylinder injection mode), high-pressure fuel needs to be supplied to a high-pressure distribution pipe connected to the in-cylinder injector.
In the in-cylinder injection mode, a high-pressure pump pressurizes fuel to raise the pressure of the fuel in the high-pressure distribution pipe to a predetermined pressure. When fuel is to be injected from the air-intake passage injector (port injection mode), the high-pressure pump stops operating to lower the fuel pressure in the high-pressure distribution pipe. However, the conventional fuel supply apparatus cannot instantaneously raise the fuel pressure to the predetermined pressure when switching from the port injection mode to the in-cylinder injection mode. Further, when switching from the port injection mode to the in-cylinder injection mode, large pulsations of the fuel pressure occurs in the high-pressure distribution pipe. This causes the injection amount of fuel to be unstable, and degrades the combustion characteristics of the internal combustion engine. To solve this problem, the fuel pressure in the high-pressure distribution pipe may be raised by actuating the high-pressure pump in the port injection mode when the fuel pressure in the high-pressure distribution pipe becomes lower than a lower limit pressure. This would keep the fuel pressure in the high-pressure distribution pipe greater than or equal to the lower limit pressure even in the port injection mode.
However, the entire amount of low-pressure fuel in the high-pressure pump would be discharged into the high-pressure distribution pipe every time the fuel pressure in the high-pressure distribution pipe becomes lower than the lower limit pressure. Thus, the high-pressure pump may excessively raise the fuel pressure in the high-pressure distribution pipe. An excessively high fuel pressure may cause fuel to leak from the in-cylinder injector or may deteriorate exhaust emission from the internal combustion engine.